1. Field of the Invention
The present invention relates to a method for continuous application of a primer coating and the continuous primed strip product therefrom. More particularly, the invention is directed to a continuous thermal deposition method for applying a primer coating to the surface of a hot-dip or electroplated zinc or zinc alloy coated steel strip at an elevated strip temperature. The continuous thermal deposition coating method of the present invention allows rapid coating of large areas of the strip surface resulting in a continuously coated product having minimal defects. The method and product of the present invention are more environmentally conscious (eco-efficient), and provide a means for improved finished sheet product quality at reduced cost. The method and product of the present invention are particularly useful for coating metal sheet used in the manufacture of certain finished and semi finished products for the metal building industry.
2. Brief Description of the Related Art
Numerous compositions and methods have been used in the past to coat sheet metal surfaces such as steel, iron, aluminum and the like, to reduce the effects of environmental degradation (weathering) and enhance appearance. In the steelmaking industry, hot-dip and electroplated zinc and zinc-alloy coatings are applied to the surface of steel sheets in continuous hot-dip coating and electroplating lines to produce corrosion resistant products, for example hot-dip galvanized, aluminized, Galvalume(copyright), galvanneal, electrogalvanized and zinc nickel electrogalvanized sheet steel products. The name Galvalume as used herein refers to a zinc hot-dip coating comprising 55% aluminum. Zinc and zinc alloy coated sheet steel products are normally formed into finished consumer products that require a durable, high quality painted surface to satisfy customer demands. Such high quality finished products range from architectural products such as roofing and siding, household products that include simple items to major appliances and furniture, and automotive products, such as interior and exterior body panels, and the like.
Currently, steel producers deliver coils of unpainted zinc or zinc alloy coated sheet to a continuous coil coater operation for production of fully painted sheet that is subsequently formed into finished consumer products. The zinc or zinc alloy coated products require steel-mill applied vanishing oil and/or chromate passivation coatings to minimize corrosion or weathering of the zinc or zinc alloy coating during transit and storage prior to painting. At the coil coater operation the steel sheet product is uncoiled, abrasively brushed, chemically cleaned, and pretreated with aqueous solutions to form tricationic zinc phosphate, hexavalent chromium, or complex oxide compounds on the zinc or zinc alloy surface. The sheet is then water rinsed and dried prior to painting with a primer coat and at least one finish coat (topcoat). Each of the numerous process steps at the steel mill and coil coater operation poses environmental, health and safety concerns. The current typical arrangement, in which steel sheet is coated with zinc or zinc alloy and a full paint system in separate steel mill and coil coater facilities, is inefficient and environmentally unfriendly, yielding hazardous waste streams that may contain persistent, bio-accumulative and toxic substances from both facilities. Combination of some all of the process steps into a single facility would significantly increase the overall efficiency as well as the eco-efficiency of the production of painted zinc or zinc alloy coated sheet products.
Several patents identify coating processes that apply paint or veneer-like coatings to metal surfaces. Patents that have addressed continuously applied coatings include U.S. Pat. No. 5,807,434 to Innes, which discloses an apparatus for two-sided paint and lubricant type coatings to elongated articles, and U.S. Pat. No. 5,919,517 to Levendusky et al., which discloses a method for applying a thermoplastic polymer resin coating to a metal strip. Neither of these patents addresses thermal deposition in applying the coating. U.S. Pat. No. 4,690,837 to Doroszkowski et al. discloses an organic coating process in which the heated surface of an article to be coated comes into contact with a coating solution bath. The articles being coated with the organic coating are immersed singly or passed sequentially through the coating solution bath. The batch processing of Doroszkowski et al. requires individual handling of each article to be coated. This increases the potential for imperfections within the finished articles, decreases the efficiency of the process, and is not practical for coating extremely long or continuous parts.
U.S. Pat. No. 5,039,360 to Brugarolas et al. discloses a method for coating a continuously moving heated metal surface using a spray gun. Spray guns impinge droplets of coating solution onto the moving metal substrate. The application and flow of enough overlapping droplets onto the moving metal surface eventually results in the formation of a uniform coating film with complete coverage of the moving metal surface. The thickness of the coating is determined by the amount of liquid coating solution applied to the strip. Brugarolas discloses a two component coating solution that cannot be recycled after mixing because of the irreversible reaction of the two components. The spray contains hexavalent chromium compounds and over spray or excess coating solution cannot be recirculated back into the spray system. Therefore, over spray and excess coating solution is a hazardous waste, and spray mists and vapors adversely impact industrial hygiene. Additionally, spray application of a coating onto the surface of a heated metal substrate creates a diffuse area of contact between the portion of the strip that has been sprayed with coating solution and the portion of the strip that has not been sprayed with coating solution. When a spray droplet impinges on a heated moving metal surface, the droplet will absorb heat from an adjacent volume of the substrate. As additional droplets impinge upon the surrounding surface, there may not be as much heat energy available in the substrate to support thermal coating deposition. The resulting uneven heat absorption may cause non-uniform deposition of the coating onto the substrate. Such spray application of coating solution cannot be used in conjunction with continuous thermal deposition because the diffuse contact area, in combination with the application of discrete droplets of coating solution, produce an uneven coating and/or coating bare spots along the surface of the heated moving metal substrate. Finally, Brugarolas teaches spray application of an acrylic organic polymer primer with a coating composition, or coating solution, containing about 20-40% by weight solids, onto a substrate heated to a temperature of between 40xc2x0 C. to 250xc2x0 C. The improved thermal deposition process of the present invention is capable of applying a uniform organic coating as thin as 0.5 micron on a continuous moving metal substrate heated to 204xc2x0 C. using a coating solution containing as little as 0.5% by weight solids.
U.S. Pat. No. 5,283,280 to Whyte et al. discloses a method for applying a polymer coating to heated objects by quenching them in a heated bath of coating solution. The heated bath of coating solution contains 2%-40% solids. Similar to Doroszkowski, the disclosed batch processing requires that each object be hung or supported individually, thereby increasing the potential for imperfections in the finished objects, decreasing the efficiency of the process and being impractical for large or continuous parts. The individual objects are placed into the bath for 15-120 seconds. The deposited coatings are disclosed to be fully dry upon removal of the object from the coating bath. In continuous thermal deposition, the coating solution does not require heating, and in some instances, the coating solution must be cooled in order to maintain a desired temperature that will maintain stability of the coating solution. Coating solutions with bath solids as low as 0.5% have been found to produce uniform organic coatings as thin as 0.5 microns. No lower limit on the contact time has been observed for the formation of a continuous thermal deposited coating, with contact times of 3 seconds being employed to deposit organic coatings. These continuous thermal deposited coatings are not dry when they are removed from the coating solution and usually require drying or curing.
Thus, each of the earlier processes discussed above has one or more shortcomings that the present invention addresses. The thermal deposition process embodied in the present invention provides for efficient, environmentally-friendly, non-contact, continuous, uniform coating of a self-supported moving sheet or web. The product embodied by the present invention is efficiently produced, more environmentally friendly, and has lower probability of handling defects than products that are manufactured using earlier coating processes.
It is an object of the present invention to provide a process for applying a continuous coating on a moving metal web by heating at least a portion of the moving metal web to a target temperature sufficient to deposit a coating on a surface thereof, and by applying a coating solution to the heated portion of the moving metal web, the applied coating solution forming the continuous coating on contact with the heated portion.
It is a further object of the present invention to provide an improved web product made by the process for applying a continuous coating to a moving metal web by heating at least a portion of the moving metal web to a target temperature sufficient to deposit a coating and applying a coating solution to the heated portion of the moving metal web to form a continuously coated web product.
It is still a further object of the present invention to provide a web product that is primed by a thermal deposition process comprising the steps of heating at least a portion of a moving unprimed web to a target temperature sufficient to deposit a primer coating and applying a primer coating solution to the heated portion of the moving web to produce a primed web surface.
It is another object of the present invention to provide an in-process coating and web product comprising a moving metal web having an unfinished portion, a finished portion, and an interim segment located between the unfinished and finished portions wherein the unfinished end of the moving metal web includes an uncoated heated portion moving toward a substantially straight line of contact with a coating solution, the interim segment of the moving metal web includes a portion having a substantially straight line of contact with the coating solution across a width of the moving metal web and a portion having an uncured and/or undried coating deposited thereon, and a portion moving through a curing and/or drying apparatus, and the finished end of the moving metal web includes the portion moving away from the curing and/or drying apparatus and including a cured and/or dried coating deposited thereon, the unfinished portion of the moving metal web continuously moving towards the finished portion of the moving metal web.
It is still another object of the present invention to provide a process that is particularly applicable for the deposition of priming compositions onto a zinc or zinc alloy coated steel substrate. Examples of such a primed product include primed galvanized sheet steel that is finish coated on a coil paint line and used as an exterior wall component, Galvalume sheet steel product that is coated with a mill-applied clear organic film and is roll formed into roof panels that receive no further organic coatings, and a primed galvanized steel product that is roll formed and fabricated into door frames that are finish painted after installation.